This invention relates generally to cathode ray tubes (CRTs) and is particularly directed to an external magnetic shield for a CRT.
Electron beam positioning on the faceplate of a CRT is controlled by a dynamic magnetic field applied to the electron beam (or beams) by means of a deflection yoke. Part of the manufacturing process of a multi-beam color CRT involves aligning the electron beams so that they are each incident upon their associated phosphor elements on the inner surface of the CRT's faceplate. Changes in applied external magnetic field, e.g., the earth's magnetic field, causes re-alignment of the electron beams from their initial positions and degraded color purity of the CRT's video image. To accommodate changes in the external magnetic field such as encountered if the CRT is moved from one location to another having a different magnetic field intensity, the CRT is provided with a magnetic shield. Even in monochrome CRTs having a single electron beam, changes in applied external magnetic field give rise to misalignment of the electron beam raster and the phosphor display area on the CRT's glass faceplate, reducing video image quality.
Magnetic shields may be either internal or external, with the former disposed on the inner surface of the CRT funnel and the latter disposed about the CRT funnel. Prior art internal and external magnetic shields are generally comprised of a soft magnetic material such as aluminum-killed (AK) steel or MOLY-PERMALLOY.TM.. An internal magnetic shield is inserted in the CRT bulb prior to evacuation and sealing of the CRT and is positioned adjacent to a conductive coating maintained at a high electrical potential on the inner surface of the CRT funnel. The internal magnetic shield sometimes introduces foreign particles in the CRT during manufacture. These contaminants frequently remain in the CRT following manufacture and tend to degrade performance in terms of video image quality and reduced the operating lifetime of the CRT.
Current external magnetic shields comprised of the aforementioned soft magnetic material are subject to rust and are frequently coated with a rust-inhibiting material such as nickel. This coating is expensive and substantially increases the cost of the external magnetic shield. In addition, application of a nickel coating frequently results in manufacturing problems because of inconsistencies in the quality of the nickel coating. Although external magnetic shields are generally more effective than internal magnetic shields, these characteristics make manufacture of CRTs with external magnetic shields more difficult and expensive.
The present invention addresses and overcomes the aforementioned limitations of the prior art by providing an external magnetic shield for a color CRT which is inexpensive, easily installed, and effectively shields the CRT electron beam (or beams) from the influence of external magnetic fields.